Allow recent pytest versions to be used with Spack (#25371)

Currently Spack vendors `pytest` at a version which is three major 
versions behind the latest (3.2.5 vs. 6.2.4). We do that since v3.2.5 
is the latest version supporting Python 2.6. Remaining so much 
behind the currently supported versions though might introduce 
some incompatibilities and is surely a technical debt.

This PR modifies Spack to:
- Use the vendored `pytest@3.2.5` only as a fallback solution, 
  if the Python interpreter used for Spack doesn't provide a newer one
- Be able to parse `pytest --collect-only` in all the different output 
  formats from v3.2.5 to v6.2.4 and use it consistently for `spack unit-test --list-*`
- Updating the unit tests in Github Actions to use a more recent `pytest` version

lib/spack/docs/conf.py

@@ -30,6 +30,7 @@
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 sys.path.insert(0, os.path.abspath('_spack_root/lib/spack/external'))
+sys.path.insert(0, os.path.abspath('_spack_root/lib/spack/external/pytest-fallback'))
 
 if sys.version_info[0] < 3:
     sys.path.insert(